PAST HISTORY OF THE INVERTEBRATES 309 



Class 2. Ophiuroidea (serpent stars) : a disk and five 



slender or branched arms, no body cavity in 



arms. Ex. Gorgonocephalus. 

 Class 3. Echinoidea (sea urchins) : hemispherical, or flat 



disk without arms. Ex. Strongylocentrotus 



drobachiensis. 

 Class 4. Holothuroidea (sea cucumbers) : long soft body 



with feathery tentacles about mouth. Ex. 



Cucumaria chronhjelmi. 

 Class 5. Crinoidea (sea lilies) : a cup with movable arms, 



attached by a stalk. Ex. Pentacrinus blakei. 



Phylum XI. Mollusca: not segmented, usually covered with a shell. 



Class 1. Acephala (clams) : no head, shell usually in two 



parts. Ex. Mya arenaria. 

 Class 2. Gasteropoda (snails) : head with tentacles, a 



single shell, usually coiled. Ex. Physa. 

 Class 3. Cephalopoda: with a circle of arms about the 



mouth, shell usually internal, free-swimming. 



Ex. Ommastrephes illecebrosus (squid). . 

 Phylum XII. Arthropoda: body with an exoskeleton, segmented, with 



body cavity, and with paired jointed appendages. 

 Class 1. Crustacea: skeleton limy, two pairs of antennae. 



Ex. Cambarus limosus. 

 Class 2. Arachnida (spiders and ticks) : no antennae, 



usually four pairs of legs. Ex. Miranda 



aurantia. 

 Class 3. Diplopoda (thousand legs) : land-living, most of 



segments with two pairs of legs each. Ex. 



Spirobolus. 

 Class 4. Chilopoda (centipeds) : one pair of legs to each 



of many somites, body flattened. Ex. Scu- 



tigera forceps. 

 Class 5. Hexapoda (insects) : one pair of antenna?, three 



pairs of legs, tracheae for respiration. Ex. 



Melanoplus femur-rubrum. 

 Phylum XIII. Chordata: with backbone -or a rod called the notochord 



in its place, gill slits in either adult or embryo, nerve 

 cord dorsal to digestive tract. 



A. The Prochordates : never having any backbone. Ex. 



Amphioxus lanceolatus. 



B. Vertebrates, with a bony or cartilaginous backbone. 

 Class 1. Pisces (fishes) : breathe by gills, body 



usually covered with scales. Ex. Perca 

 flavescens. 



